Mitosis: New techniques expose surprises in cell division

Date:

September 12, 2011

Source:

California Institute of Technology

Summary:

Researchers have obtained the first high-resolution, three-dimensional images of a cell with a nucleus undergoing cell division. The observations, made using a powerful imaging technique in combination with a new method for slicing cell samples, indicate that one of the characteristic steps of mitosis is significantly different in some cells.

Share:

Total shares:

FULL STORY

A "regular" spindle configuration (top) with all chromosomes attached to microtubules, and the O. tauri spindle (bottom) with chromosomes bundled together and attached to just two short, incomplete microtubules.

Credit: Caltech

A "regular" spindle configuration (top) with all chromosomes attached to microtubules, and the O. tauri spindle (bottom) with chromosomes bundled together and attached to just two short, incomplete microtubules.

Credit: Caltech

Researchers at the California Institute of Technology (Caltech) have obtained the first high-resolution, three-dimensional images of a cell with a nucleus undergoing cell division. The observations, made using a powerful imaging technique in combination with a new method for slicing cell samples, indicate that one of the characteristic steps of mitosis is significantly different in some cells.

During mitosis, two sets of chromosomes get paired up at the center of the cell's nucleus. Then hollow rods of proteins called microtubules, which make up a cellular structure called the spindle apparatus, grab on to the chromosomes and essentially pull each set away from the center in opposite directions, so that both daughter cells end up with a full copy of the genetic material. Typically, in the cells of plants, fungi, and many animals, one or more microtubules attach to each chromosome before the spindle will separate the sets of chromosomes from each another.

But when the Caltech researchers observed this step using their new technique, what they saw was not business as usual for a dividing cell. "We've found the first clear example of a cell where there are fewer microtubules used than chromosomes," says Grant Jensen, professor of biology at Caltech and a Howard Hughes Medical Institute investigator. The group's findings appear online in Current Biology and will be published in the September 27 issue of the journal.

Jensen's group is one of just a few in the world that uses electron cryotomography (ECT) to image biological samples. Unlike traditional electron microscopy -- for which samples must be dehydrated, embedded in plastic, sectioned, and stained -- ECT involves plunge-freezing samples so quickly that they become trapped in a near-native state within a layer of transparent, glasslike ice. A microscope can then capture high-resolution images of the sample as it is rotated, usually one degree at a time.

One limitation of ECT is that samples cannot be thicker than 500 nanometers -- otherwise the electron beam cannot penetrate the sample sufficiently. Therefore, ECT studies have focused on small bacteria and viruses. But Jensen's group wanted to extend the technique to observe eukaryotic cells, which are typically much bigger. So they located the smallest known eukaryote, Ostreococcus tauri, and imaged it with ECT.

The next step was to observe the important process of cell division in a eukaryote. But even tiny O. tauri exceeds the 500-nanometer limit when it is undergoing mitosis, since cells are essentially twice as big as usual when they're dividing. So the researchers needed a way to cut the frozen sample into slices, a process called cryosectioning.

"In the past when people have tried this, the sections have come off sort of like snowflakes -- the material has gotten crushed," Jensen says. But Caltech electron microscopy scientist Mark Ladinsky has developed a highly successful new technique for cryosectioning samples. He slices them at about -150°C, using a special machine and a diamond knife. Then he carefully removes the slices from the knife using a micromanipulator. The technique has enabled the researchers to look at slices through dividing cells in a near-native, hydrated state.

With the new imaging and sectioning techniques working together, a former postdoctoral scholar in Jensen's lab, Lu Gan, was able to make detailed observations of mitosis in O. tauri, a cell with 20 chromosomes. Contrary to expectations, Gan observed nowhere near 40 microtubules attached to the two sets of chromosomes during mitosis; instead, he found only about 10 small, incomplete microtubules. This suggests that the chromosomes may link together to form some kind of a bundle that can then be segregated all at once by a smaller number of microtubules.

Previous studies, dating back to the 1970s, claimed to have found unicellular eukaryotic cells with fewer microtubules than chromosomes. However, those cells were chemically fixed and stained -- a process that can easily damage the cells -- casting doubt on the claims. But with their new imaging and sectioning techniques, the Caltech researchers feel confident that they have indeed imaged such a cell.

Their success bodes well for future studies. "Our work with O. tauri shows that we might be able to get high-resolution, three-dimensional images of other eukaryotic cells, which are much larger than bacteria," Jensen says. "We've even moved on to try to image some human cells using the same process."

The group's report, "Organization of the Smallest Eukaryotic Spindle," was supported in part by a grant from the National Institutes of Health and a fellowship from the Damon Runyon Cancer Research Foundation. The ECT imaging was made possible by a gift from the Gordon and Betty Moore Foundation.

July 31, 2015  Resettlement projects in the Amazon are driving severe tropical deforestation, according to new research. Widely hailed as a socially responsible and 'innocuous' strategy of land redistribution, ... read more

July 29, 2015  Viewing aquarium displays led to noticeable reductions in blood pressure and heart rate, a research team found in the first study of its kind. They also noted that higher numbers of fish helped to ... read more

July 31, 2015  A new study examines how consuming the concentrated extract of thylakoids found in spinach can reduce hunger and cravings. Thylakoids encourage the release of satiety hormones, which is very ... read more

July 30, 2015  The behavior of fruit flies, which are commonly used in laboratory experiments, is altered by electric fields, new research shows. The research indicates that the wings of the insects are disturbed ... read more

July 31, 2015  Starvation early in life can alter an organism for generations to come, according to a new study in nematodes. The epigenetic effects are a 'bet-hedging strategy.' Famine survivors are smaller and ... read more

July 31, 2015  The humble butterfly could hold the key to unlocking new techniques to make solar energy cheaper and more efficient, pioneering new research has shown. By mimicking the v-shaped posture adopted by ... read more

Dec. 10, 2014  Scientists have explored the role of three molecules in controlling the process of cell division in a bid to gain new insight into the transmission of vital signals from a cell's exterior to its ... read more

Dec. 2, 2014  Scientists have revealed that the dsRNAs and Protein Kinase R (PKR) regulate division of mammalian cells. This finding will provide important clues to understanding the process of tumor formation and ... read more

Jan. 23, 2014  Losing or gaining chromosomes during cell division can lead to cancer and other diseases, so understanding mitosis is important for developing therapeutic strategies. New research focused on one ... read more

Dec. 15, 2013  Resolving a fundamental question in cell biology and showing off the powers of new high-resolution 3-D imaging, scientists have discovered where the Golgi apparatus, which sorts newly synthesized ... read more